Automatic number identification (ANI) is a feature of a telecommunications network for automatically determining the origination telephone number on toll calls for billing purposes.

In telephony, ANI service was originally created by AT&T Corporation for internal long distance charging purposes,[1][2] eliminating the need for telephone operators to manually request the number of the calling party for a toll call.

Modern ANI has two components: information digits, which identify the class of service,[3] and the calling party billing telephone number.

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Modern toll-free telephone numbers, which generate itemized billing of all calls received instead of relying on the special fixed-rate trunks of the Bell System's original Inward WATS service, depend on ANI to track inbound calls to numbers in special area codes such as +1-800, 888, 877, 866, 855 and 844 (United States and Canada), 1800 (Australia) or 0800 and 0808 (United Kingdom).

ANI is different, conceptually and technically, from caller ID service. A caller's telephone number and line type are captured by ANI service even if caller ID blocking is activated. The destination telephone company switching office can relay the originating telephone number to ANI delivery services subscribers. Toll-free subscribers and large companies normally have access to ANI, either instantly via installed equipment, or from a monthly billing statement. Residential subscribers can obtain access to ANI information through third party companies that charge for the service.[4]

ANI is generally not transmitted when a call is operator assisted; only the area code of the last switch to route the call is sent.

ANI is used to provide automatic number announcement, an internal test facility for telephone maintenance workers. The service, which is not advertised to the public, allows an installer to identify an individual line by using it to dial a special unpublished number in a range reserved for testing purposes (such as 958-xxxx in much of North America).

A related service feature available to private branch exchange subscribers is the Dialed Number Identification Service (DNIS), a code indicating the number requested by the caller. With the information, a service provider can have several toll-free numbers directed to the same call center and provide unique service based on the number dialed. DNIS can also be used to identify other call routing information. For example, toll-free service can be configured to send a specific DNIS number that is assigned to callers from geographic regions based on city, area code, state, or country.